1. Field of the Invention
The present invention relates to a current lead, and more particularly to a current lead of a superconducting magnet for magnetic resonance.
2. Description of the Prior Art
In a superconducting type of magnetic resonance system, a superconducting magnet is magnetized by a current provided gradually by a magnet power supply in a low temperature environment, i.e. at the temperature of the liquid helium of an absolute temperature value of 4.2 K (−269°), so as to build up the magnetic field; once the magnetization is accomplished, since the coil is at a superconducting status, there is no need to provide a power supply to the superconducting magnet, as long as a stable superconducting environment is maintained, therefore a highly stable and uniform magnetic field is achieved.
An important condition for a superconducting type of magnetic resonance system to be successfully magnetized and to work in a stable manner is to establish and maintain a stable superconducting environment. Once the superconducting environment is damaged, this leads to the quenching of the superconducting magnet. After quenching, the economic losses due to restarting the refrigeration, magnetization as well as those due to the downtime of the magnetic resonance system are very large.
The superconducting magnet requires for its magnetization a specialized high precision power supply system, which is placed in an environment of room temperature, and it supplies the power to the superconducting magnet in the superconducting environment through current leads. Heat can be conducted to the superconducting magnet via the current leads, thus the superconducting environment is affected. At the same time, when it is magnetized or demagnetized, a huge current of a few hundred amperes will pass through the current leads, and due to the resistance in the current leads themselves, the ohm heat resulting from the resistance under the effects of the huge current can also be conducted to the superconducting magnet, thus affecting the superconducting environment.
In the prior art, usually two types of current leads are used to supply the power to the superconducting magnet; one is a detachable current lead, and the other is a fixed current lead. When using the detachable current leads, the detachable current leads are electrically connected to the superconducting magnet only when the superconducting magnet needs to be magnetized or demagnetized, and they are separated from the superconducting magnet when the superconducting magnet is in a stable superconducting status. Since the detachable current leads are not electrically connected to the superconducting magnet for most of the time, external heat is effectively prevented from being conducted to the superconducting magnet via the current leads; however such detachable current leads have a complex structure and they require handling by an experienced person when they are connected to or detached from the superconducting magnet. When the fixed current leads are used, the fixed current leads are always connected to the superconducting magnet. In this case, it is unavoidable for the external heat to be conducted to the superconducting magnet via the current leads, however such fixed current leads are convenient in operation, and as the refrigeration capability of refrigeration devices is being improved, the shortcoming caused by the heat conduction is compensated to a certain degree, therefore in the superconducting type of magnetic resonance systems the fixed current leads are increasingly used in practical commercial applications.
Referring to FIG. 1, in a prior art connection device of the fixed current leads, a superconducting magnet 70 is submerged in liquid helium within a liquid helium container 60, the current leads 40 are provided in a turret tube 50 in communication with the liquid helium container 60, and the current leads are electrically connected to an external power supply system and the superconducting magnet 70 respectively to supply the power to said superconducting magnet 70. Said current leads 40 comprise the leads used respectively as a positive current lead and a negative current lead. A superconducting switch 30 is connected between the positive current lead and the negative current lead to control the superconducting magnet 70. A cold head 10 is provided in a side tube (Sidesock) 20 in communication with the liquid helium container 60, to be used for refrigeration. As described above, this current lead connecting device would conduct external heat to the superconducting magnet 70 via the current leads 40 and the ohmic heat resulting from the strong current passing through the current lead 40 would also be conducted to the superconducting magnet 70, leading to the evaporation of the liquid helium due to the heat in the liquid helium container 60 and the damage to the superconducting environment. In addition, in such a current lead connecting device, the current leads 40 and the cold head 10 are provided in the turret tube 50 and the side tube 20 respectively, leading to a complicated structure for the current lead connecting device.
FIG. 2 shows an improved current lead connecting device, in which the differences between it and the current lead connecting device shown in FIG. 1 are: the current lead 40 is only used as the positive current lead and a side wall of the turret tube 50 is used as the negative current lead. The advantage of this improved current lead connecting device is the reduced number of current leads 40, however although a part of the heat conducted from outside to the superconducting magnet 70 through the current lead 40 is reduced, the difficulty for designing and producing the turret tube 50 is increased, because not only the heat conduction property but also the mechanical properties as well as the electric conduction properties of said turret tube 50 need to be considered. In the same way as the current lead connecting device shown in FIG. 1, the current lead 40 and the cold head 10 of this improved current lead connecting device are provided in the turret tube 50 and the side tube 20 respectively, leading to the complicated structure of the current lead connecting device.